Under / Over Correction

[Alkaid]

Hi all,

Please can anyone explain what is meant by under correction and over correction when star testing the optics of a telescope?

Thanks Steve

[xyz]

I believe that with reference to primary mirrors. Undercorrected means that the curve of the mirror is less parabolic than it should be (ie. it is more like a sphere). Overcorrected means that the parabola is too steep and is more parabolic than ideal.

[NGC 1502]

Under and over correction is deviation either side of theoretical perfection in

optical components.

Regards, Ed.

[hunterknox]

Under and over correction is deviation either side of theoretical perfection in

optical components.

So if something's over corrected it's better that perfect?!

They're both forms of spherical aberration. Have a look at the picture on the right of the page on Wikipedia:

Spherical aberration - Wikipedia, the free encyclopedia

[SiriusB]

I believe that with reference to primary mirrors. Undercorrected means that the curve of the mirror is less parabolic than it should be (ie. it is more like a sphere). Overcorrected means that the parabola is too steep and is more parabolic than ideal.

I believe in essence this answer is correct at least as far as primary mirrors go.

A parabolic mirror has an eccentricity of e=1

An undercorrected mirror called an oblate spheroid has e<1

An overcorrected mirror called a hyperboloid has e>1

Its all down to trigonometry- if you have a good grasp of this its going

To be easier to understand. Something to do with conic sections i believe?

Alas we did nothing about this in maths lessons, so I’ve always struggled

To get my head round the concepts.

What's the practical application?

Well a parabolic mirror should focus on & of axis rays to a common point.

A slightly under or over corrected mirror will focus off axis rays short or long

Compared to on axis rays (not sure which way round it is)

So a parabolic mirror should focus more sharply (at least on axis)-

I understand undercorrected mirrors have better off axis performance,as it reduces coma,an unavoidable byproduct of the parabolic mirror.

As far as under or over corrected objective lenses go?..ask someone else.

Hope that helps,Bet you wish you hadn't asked!

[NGC 1502]

So if something's over corrected it's better that perfect?!

Well no, if an optical component is overcorrected then it is not

perfect. As I said, under OR over correction is deviation either

side of theroreticlal perfection, ie, they are both "incorrect"

Best regards, Ed.

[hunterknox]

I know what you mean NGC, I'm just kidding you.

Mountain man - if it's still unclear here's my best shot at a simple explanation:

In both under or over correction the light that hits the lens or mirror is not all brought to focus at the same point .

If the lens or mirror is under corrected then the light from the outside of the lens/mirror is brought to focus closer to the lens/mirror than light from the centre.

If the lens/mirror is over corrected then the light from the outside of the lens/mirror is brought to focus further away from the lens/mirror that light from the centre.

In practice both under and over correction make it impossible to get the sharpest focus possible, so the image looks slightly soft.

[sixela]

I believe that with reference to primary mirrors. Undercorrected means that the curve of the mirror is less parabolic than it should be (ie. it is more like a sphere). Overcorrected means that the parabola is too steep and is more parabolic than ideal.

Can't be "more parabolic [paraboloidal, actually]" than the ideal paraboloid (which always has a conic constant of 1). When it's overcorrected it becomes hyperboloidal, when it's undercorrected it's ellipsoidal or spherical, and when it's just at the cusp between an ellipsoid and a hyperboloid it's just the "right" paraboloid.

[Alkaid]

Got it guys, thanks a lot! Now I understand.

I believe you can tell which way the optics fall by comparing the de-focused star images at high power, then going through the focus point to the next similar de-focused image.

By that I mean, insert eyepiece, look at a star. Rack out focuser until diffraction rings are seen. Then focus but keep racking the focuser out so that you go through the focal point until the diffraction rings are seen again on the other side of the focal point.

Correct me if this is total nonsense, but I read somewhere that a perfectly corrected scope will display the same diffraction rings each side of focus. You can tell if your optics are under or over corrected by comparing the two de-focused star images. However, I am still learning how to do this. All good fun!

Cheers all!

Steve

[NGC 1502]

Correct me if this is total nonsense, but I read somewhere that a perfectly corrected scope will display the same diffraction rings each side of focus. You can tell if your optics are under or over corrected by comparing the two de-focused star images. However, I am still learning how to do this. All good fun! Steve

Yes thats right, but theoretical perfection is unatainable. Most

telescopes, even excellent ones, will show some difference when

examining the out of focus image carefully, and if you don't know

that, it could cause you to be unhappy with a nice instrument.

Thats what happened to me, until someone explained it.

Some time ago, when I had a Skywatcher 200P dob, I was a bit

unhappy when I did the out of focus test, and saw a difference

either side of focus. A clubmate explained to me what I've just

said, and I was releived to hear it. Later another clubmate with

an OO dob with an uprated mirror, did a side by side comparison

looking at Saturn, in steady seeing conditions. We could not see

any difference in the view, and my clubmate is an honest person,

if he had seen a difference, he'd have politely said so.

It's possible, that under really excellent conditions, at very high

power the uprated mirror would have given a better view.

Just relax, and enjoy the view ! Cheers, Ed.

Edit : Hunterknox - sorry if I took you too seriously Ed.

[John]

FWIW a large proportion of chinese manufactured achromatic refractors are undercorrected a little. It does not affect the views, other than the star tests under very good seeing conditions.

Critical star testing is difficult to do in the UK (outside of a lab) because our atmosphere is rather unsteady much of the time which has an adverse affect on getting clear definition of diffraction rings either side of focus.

It's easy to get paranoid about this (I know I have at times) but, as ED says above, it's the views you get that count !.

[Alkaid]

Yup, good advice guys and a fair point about actually observing and enjoying the views in focus rather than getting too scientific about star testing itself.

However, understanding on this concept improved thanks to your replies so thanks for the heads up

Clears skies!

Steve

[umadog]

Note, however, that even a perfect parabolic mirror will exhibit coma off-axis:

Coma (optics) - Wikipedia, the free encyclopedia

You need an extra optical element to correct this, such as a paracorr. Other designs, such as SCTs, would also exhibit coma were it not corrected.